Combine the size with the fact that they are semi transparent and live in hair follicles and sebaceous glands, both of which are essentially under the skin, and that's why you don't generally see them.

0.3mm in length, not diameter. Yes, you can see them, but they just look like dead skin/gunk to our eyes.

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Gallery with visible accumulation of mites on hairs

Maybe we do see them, but mistake them for bits of skin or dirt.

Agreed. 1/64th of an inch is .016". An average human hair is .003" in diameter, that would be as long as five hairs wide. You wouldn't even need a magnifier to see the largest ones.

I learned recently that machinists used to use fractions to indicate tolerance, like a dimension given as 5/16 could be ±1/32 of an inch, because anything less than 9/32 would round to 4/16, and anything more than 11/32 to 6/16.